You can buy a universal walking foot for your cheap Wal-mart machine. I did. It makes straight line quilting easier. I also bought a FMQ foot for it. They were about $12 each. I can even do a sort of respectable FMQ now. Quilting gloves ($4 at Joann's) really helped, too.
Give it a try. I'm pretty sure Wal-mart doesn't sell a junkier machine than mine and I can do at least 52 x 72. That's the biggest I've tried.